IR: Refactoring and introduction of explicitly names "statement nodes"

[mlange05]

Note: This PR technically introduces an externally API-changing change, as it renames ir.Intrinsic -> ir.GenericStmt

This PR refactors the loki.ir.nodes source into a dedicated sub-package and introduces a new set of statement nodes that all inherit from loki.ir.nodes.GenericStmt. These replace previous ir.Intrinsic nodes (a misnomer from the early days 😉 ), which in turn allows us to refer to specific statement nodes explicitly by type (eg. ir.ImplicitStmt or ir.ContainsStmt). It is worth pointing out that I've only specialised CONTAINS, IMPLICIT, CYCLE, RETURN, GO TO, PUBLIC, PRIVATE thus far, but others could follow.

Pleas note also that the latter two now exist as dedicated node types, but I've not changed (yet) the internal mechanics of the Module that use this information.

In some more detail:

• Moved IR utilities (including dataclass_strict to loki.tools
• Separated loki.ir.nodes into sub-package with sources for abstract_nodes, internal_nodes and leaf_nodes
• Renamed loki.ir.Intrinsic => loki.ir.GenericStmt
• Added source for stmt_nodes to loki.ir.nodes and implemented ImplicitStmt, ContainsStmt, ReturnStmt, CycleStmt, GotoStmt, PrivateStmt and PublicStmt
• Adjusted used of generic statement nodes in both code and test base
• Consolidated some statement tests in ir.nodes.tests to capture new node types and avoid JIT
• Some more import massaging in a test files that I had to touch anyway

[github-actions]

Documentation for this branch can be viewed at https://sites.ecmwf.int/docs/loki/656/index.html

[codecov]

Codecov Report

❌ Patch coverage is 95.72414% with 31 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.38%. Comparing base (b711b95) to head (da12fe7).

Files with missing lines	Patch %	Lines
loki/ir/nodes/stmt_nodes.py	90.21%	9 Missing ⚠️
loki/ir/nodes/abstract_nodes.py	92.92%	8 Missing ⚠️
loki/ir/nodes/internal_nodes.py	95.53%	8 Missing ⚠️
loki/frontend/fparser.py	95.34%	2 Missing ⚠️
loki/frontend/omni.py	91.30%	2 Missing ⚠️
loki/tests/test_sourcefile.py	93.10%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #656      +/-   ##
==========================================
- Coverage   96.39%   96.38%   -0.02%     
==========================================
  Files         266      271       +5     
  Lines       46418    46567     +149     
==========================================
+ Hits        44745    44883     +138     
- Misses       1673     1684      +11     

Flag	Coverage Δ
lint_rules	96.38% <100.00%> (-0.03%)	⬇️
loki	96.38% <95.68%> (-0.02%)	⬇️

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